To a physicist, a “body” is any object with mass. Depending upon the problem at hand, it can be an atom, a block of wood, a person, a plane, a train, an automobile, a comet, a planet, a star, a black hole, even an entire galaxy. If we want to calculate the trajectories of two bodies moving under their mutual influence—for example, a single planet orbiting a star—that is referred to as a two-body problem. There are reasonably simple equations that can define the state—the positions and velocities of both bodies—of that system at any time.
